1. Field of the Invention
The present invention relates to a parabolic reflector antenna and, more particularly, to a parabolic reflector antenna for signal transmission between the radio communication base station (BS) and the mobile terminals.
2. Related Art
The parabolic reflector antenna is a high gain antenna with orientation that is mostly used in radar systems, microwave systems and satellite systems. Its reflector is shaped into one that can readily collect radio signals. It is also termed open antenna. Since the dimension of the reflector is much larger than the signal wavelength, so it can be applied to electromagnetic radiation. The parabolic reflector antenna comprises two main components: one is a parabolic reflector and the other is a feedback active unit. Basically, the active unit is the original antenna for signal feedback. Usually, the electromagnetic energy is directed toward the reflector using dipoles, dipole arrays, and waveguides. The reflector is a passive device with no power. It is solely used to focus and direct the electromagnetic energy supplied from the original antenna by reflecting and to spread the energy into space.
The parabolic reflector is a dish disk, so it is also called the dish antenna. The function principle of the parabolic reflector can be elucidated in terms of the geometric properties of parabolas. Please refer to FIG. 1. In the x-y coordinate system, a parabolic curve can be expressed by EQU y.sup.2 =4fx, (1)
where f is the distance from the parabolic vertex to the focus F of the parabola. According to the geometric characters of parabolas, the sum of the distances from any point (such as A, B and C) on the parabola to the focus F and to points on the line y.sub.1 -y.sub.2 parallel to the directrix is a constant. That is, EQU FA+AA'=FB+BB'=FC+CC'=K, (2)
where K is a constant. The parabolic reflector is a disk-like curved surface formed by rotating a parabola about the x-axis, which is called the paraboloid. The paraboloid is often used in car headlights for focusing light. If the electromagnetic energy is radiated from the focus F and reflected by the parabolic reflector, all simultaneously emitted radiation wave fronts reach the line Y.sub.1 -Y.sub.2 at the same time. The distances traveled by all waves are equal, and the wave shapes and phases on this line are the same. Therefore, this reflector can focus the electromagnetic energy into a beam along the x-axis and form a high gain directional radiation.
In particular, the feedback device of the parabolic reflector antenna is the actual supplier of electromagnetic energy radiation, and is thus called the primary antenna. The primary antenna is positioned on the focus of the parabolic reflector for obtaining the best emission or reception effects. Aside from dipole antenna, the primary antenna often uses waveguides to form a horn feed in the microwave frequencies.